The suitability of radioallergosorbent test (RAST) inhibition to quantify occupational exposure to rat urinary aeroallergen (RUA) has been assessed. When using a constant pool of rat allergic sera, the reproducibility of the assay over 1 year was comparable to that reported for other immunoassays; at 50% RAST inhibition the inter-assay coefficient of variation (CV) was 7.0% and the intra-assay CV was 3.0%. The assay was highly specific for rat urine; mouse urine was 1100-fold less potent at inhibiting the rat urine RAST system. Significant inter-assay variation in the 'high' control was not due to batch variation and was relatively small when compared with the variation in RUA concentrations in the occupational environment. Measurement of workplace RUA exposure demonstrated that those directly involved in the care of rats experienced the highest RUA exposure of the nine occupational groups studied (animal technicians GM = 23.10 micrograms/m3), dead animals (e.g. post mortem GM = 1.60 micrograms/m3, scientists GM = 0.67 microgram/m3) and rat tissue (e.g. slide production GM = 0.04 microgram/m3). In view of the complexity of rat allergens, RAST inhibition is an appropriate method for the quantification of occupational exposure to rats.